Apparatus and method for electrolytic deposition of metal layers on workpieces

ABSTRACT

The present invention is related to an apparatus and a method for the electrolytic deposition of a zinc or zinc alloy layer on a workpiece, the apparatus comprising a container to hold the metal plating bath divided into at least two compartments with an electrode assembly in the container, the assembly comprises in a first compartment a soluble anode to provide ions of the at least one metal to be deposited and a cathode corresponding to the workpiece to be metal plated and in an anolyte compartment separated from the first compartment by an ion exchange membrane comprises an insoluble anode and wherein the electrode assembly in the compartments are connected by an adjustable power supply.

FIELD OF THE INVENTION

The present invention relates to an apparatus for the electrolytic deposition of a zinc or zinc alloy layer on a workpiece utilising a special anode assembly comprising a soluble and an insoluble anode. The invention also relates to a process for zinc or zinc alloy layer deposition from an electrolytic plating bath using such apparatus.

BACKGROUND OF THE INVENTION

Electrolytic metal deposition requires the use of at least one electrode which is cathodically (the cathode) polarized to another electrode, which is anodically polarized (the anode).

Depending on the kind of electrolyte used for the metal deposition, such anode could either be a soluble anode, which upon anodic oxidation releases ions of the metal it is made from into the bath solution or it could be designed as an insoluble anode, sometimes referred to as inert anode.

Application of an insoluble anode needs to utilize another anodic reaction than the metal's dissolution in order to allow for a current flow. Commonly, such alternative anodic reaction is the solvent's decomposition. The most applied solvent is water. Therefore, the most common anodic reaction in galvanic deposition with insoluble anodes is oxygen evolution due to oxidation of the oxygen in the water molecule, which can be represented by the following formula:

6 H₂O

O₂↑+4 e ⁻+4 H₃O⁺

for acidic aqueous solutions.

The term insoluble anode is commonly used not to express that absolutely no material of the anode is anodically oxidized and thus dissolved, but the majority of the current is utilized to generate oxygen. However, insoluble anodes made from e.g. titanium or platinised titanium are virtually not oxidised and do not release platinum or titanium ions into the bath solution, thus remain stable.

Soluble anodes in contrast utilize the majority of the applied current to oxidize the anode material forming the ions of the anode material, e.g. Zn²⁺ ions from soluble zinc anodes.

For example, if the anode material is zinc, zinc ions are formed by the anodic oxidation reaction, which can be represented by the following formula:

Zn

Zn²⁺+2e ⁻

Only a minor amount of current at the soluble anode is utilized for other reactions, e.g. undesired water decomposition to form oxygen.

The application of insoluble anodes is more common in alkaline electrolyte compositions whereas soluble anodes are mostly employed in acidic bath compositions.

One important advantage of insoluble anodes is that they are dimensionally stable and do not require anode material replenishment. On the other hand with insoluble anodes the metal being deposited at the cathode needs to be replenished frequently into the bath by means of the corresponding metal ion source, most often a metal ion salt of the metal to be deposited at the cathode.

The advantage of soluble anodes is that the metal being deposited on the cathode can be replenished into the bath from the anode material. This working mode requires new anode material to replenish the dissolved metal.

The anodic current efficiency of the metal dissolution reaction is usually almost 100%, e.g. acidic zinc electrolytes when applying common anodic current densities and normal working conditions.

In contrast, the cathodic current efficiency is usually well below 100% due to additional processes like reduction of certain organic bath components and water decomposition, particularly at high current densities.

The higher current efficiency at the anode compared to the cathode results in an increase of the metal ions dissolved from the soluble anode material because less metal ions are reduced at the cathode than dissolved from the anode. Ideally, the anode and cathode current efficiency are the same because under these conditions all metal ions dissolved from the anode are reduced at the cathode and the ion concentration in the plating bath remains constant.

However, in cases with different anodic and cathodic current efficiency, the metal concentration depends on the working conditions and side reactions and is more difficult to control.

A common solution to reduce the increasing metal ion concentration in plating operations is to frequently dilute the bath and thus control the metal content.

This operation is inefficient as it results in discharge of certain materials into the waste-water, thus creating an environmental burden, binds working force and leads to operation down times.

Also, utilization of insoluble anodes in acidic electrolytes which contain halides like chloride is technically limited due to the anodic oxidation of chloride into toxic chlorine gas at the anode.

It would, therefore, be desirable to provide an apparatus and a method for metal plating which combines the advantages of both, the system applying an insoluble anode as well as equipment utilising soluble anodes.

SUMMARY OF THE INVENTION

The apparatus according to the present invention is characterised in that it contains two types of anodes, soluble and insoluble anodes which allow adjustment of the current efficiency at the soluble anodes to correspond to the cathode efficiency at the cathode, i.e. the workpiece. Thereby, the concentration of metal ions dissolved from the soluble anodes corresponds to the amount of ions deposited on the cathode by an electrochemical reaction and the amount of metal ions in the plating bath solution remains constant.

The apparatus is used for electrochemical deposition of zinc and zinc alloys from metal plating baths and results in deposits of high uniformity. The apparatus is particularly suited for deposition of zinc alloys exhibiting a homogeneous deposit having a constant ratio of metals in the alloy.

The use of ion exchange membranes as such is known in the art. Document WO 01/96631 A1 relates to a plating process utilising a cationic ion exchange membrane and an insoluble anode to prevent formation of anodic breakdown products in the bath.

The present invention in contrast is not directed to a method to prevent the formation of breakdown products or the cementation of nobler metals, but to control the metal ion concentrations by a special electrode assembly enabling metal deposition with constant deposition results and homogeneous deposits.

Document DE 42 29 917 C1 relates to operation of an electrolytic bath for metal coating a substrate which uses a secondary anode cell containing alkaline or ammonium solutions and is sealed with a membrane to allow the passage of alkali or ammonium ions into the plating solution. Acid needs to be added to the bath to compensate for the pH rise which occurs as hydrogen is evolved at the cathode. This method is disadvantageous because it requires pH control.

Using an alkaline anolyte results in migration of potassium ions (source: KOH) from the anolyte compartment into the plating bath. This results in the undesired accumulation of the potassium ions in the plating bath which is as undesired as the accumulation of zinc ions.

JP S56 112500 A describes a method for the metal plating with the aim to stabilize the composition of a plating bath and thus enable operation for a long period by providing an insoluble auxiliary anode, in addition to a soluble anode, in an aqueous acid solution via a cationic ion exchange membrane in a plating tank, and flowing a part of current thereto for plating. This document fails to teach the specific advantages of the setup according to the present invention for zinc and zinc alloy plating methods.

JP 2006 322069 A relates to an electrogalvanizing method which is performed in a plating bath provided with an insoluble anode isolated by a cationic ion exchange membrane and a soluble anode. The substrate to be plated is successively washed by water in washing tanks, and the plating liquid from the plating bath is recovered. Due to the isolation of the insoluble anode, the decomposition of salt and organic matter on the insoluble anode is prevented. The problem to be solved is to avoid decomposition by oxidation at the anode and thus differs from the object of the present invention. This becomes apparent by the substantially different setup of the electrode assembly.

It is, therefore, aim of the present invention to provide a metal plating method which avoids any accumualtion of metal ions in the plating bath at all.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows an embodiment of an apparatus according to the present invention comprising in a first compartment a soluble anode, the workpiece as a cathode and in a second compartment an insoluble anode. The first and the second compartment are divided by an ion exchange, e.g. cationic exchange membrane.

FIG. 2 shows an embodiment of an apparatus according to the present invention, wherein two power supplies (6 a) and (6 b) are used in order to distribute the current soluble zinc anode (2) and insoluble anode (5).

FIG. 3 shows an embodiment of an apparatus according to the present invention which additionally contains a second soluble anode (2′) to provide metal ions for alloy deposition.

DETAILED DESCRIPTION OF THE INVENTION

It was a surprisingly found that an apparatus comprising in a metal plating bath for zinc and zinc alloy plating at least one soluble zinc anode to provide a source zinc ions to be deposited and the cathode and in an anolyte compartment an insoluble anode, wherein the plating bath and the anolyte are separated from each other by an enclosure bearing on at least a portion of the enclosure an ion exchange membrane, which preferably is a cationic exchange membrane (also denoted cationic membrane).

The insoluble anode can be selected from the group consisting of titanium anodes, platinized titanium anodes, ceramic coated anodes and carbon based anodes like graphite anodes.

The plating bath contains the at least one soluble zinc anode and the cathode, which corresponds to the workpiece to be metal plated.

In case an alloy of more than one metal is to be plated, the first compartment can contain two or more soluble anodes made from to the metals to be plated. For example, if a zinc nickel alloy is to be plated the first compartment can contain a first soluble anode made of zinc and the second soluble anode made of nickel.

The second soluble anode in addition to the soluble zinc anode can alternatively be selected from at least one of the group consisting of nickel, cobalt, iron, chromium, cadmium, tin, copper, silver, gold, platinum, palladium, manganese, ruthenium, or rhodium, iridium, osmium, rhenium, tungsten, molybdenum, vanadium, indium, bismuth, antimony, selenium, germanium, gallium and tantalum, niobium. Nickel and iron are preferred as the second soluble anode material, nickel being particularly preferred.

Alternatively, if a zinc alloy is to be plated, the second metal can be provided via its salt. For example, a typical setup would be an apparatus comprising in the plating bath a soluble zinc anode, nickel sulphate as the nickel source, and complexing agents, and in the anolyte an acid, preferably sulphuric acid in an aqueous solution.

The apparatus according to the present invention can preferably be applied for the deposition of zinc nickel binary or ternary alloys. The deposition of binary zinc nickel alloys is particularly preferred. A typical setup for the deposition of a zinc nickel deposit is shown in FIG. 1. The container (1) comprises in the plating bath (1 a) the soluble anode (2) made from zinc which generates zinc ions when electric current is provided by the adjustable power supply (6). The plating bath (1 a) further contains hydrochloric acid or sulfuric acid, nickel chloride as nickel ion source, typically potassium chloride to increase the conductivity and auxiliary components like wetting agents, buffers and brightening agents. The plating bath (1 a) also contains the cathode (3) which is the workpiece to be metal plated. The anolyte compartment is divided from the first compartment by suitable separation means. The means bears on at least a portion thereofan ion exchange membrane (4 a). The membrane (4 a) preferably is a cationic ion exchange membrane in case the plating bath is acidic in pH. The separation by suitable means can for example be an enclosure made of polymers chemically resistant to the plating bath, e.g. made from polypropylene.

The anolyte also holds the insoluble anode (5). The current flow is adjusted by the adjustable power supply (6) in such a way that essentially all zinc ions dissolved from the soluble anode (2) are deposited onto the cathode (3) as zinc nickel alloy deposit. The nickel ions (3) deposited at the cathode as nickel metal are replenished by adding the corresponding nickel salt, i.e. nickel chloride.

A typical zinc nickel electroplating bath suitable for use in the apparatus according to the present invention contains zinc ion concentrations ranging from 0.1 to 100 g/l, preferably from 5 to 60 g/l and even more preferred from 20 to 35 g/l.

The electroplating bath of the present invention further comprises nickel ions with concentrations ranging from 0.1 to 60 g/l, preferably from 10 or 50 g/l and even more preferred from 25 to 35 g/l. Sources of nickel ions which can be used comprise inorganic salts of nickel and organic salts of nickel. In one embodiment, the nickel source includes one or more of nickel hydroxide, nickel sulfate, nickel carbonate, ammonium nickel sulfate, nickel sulfamate, nickel acetate, nickel formiate, nickel bromide, nickel chloride.

In one embodiment, the zinc ions and the nickel ions are present at concentrations sufficient to deposit a zinc-nickel alloy comprising a nickel content from 3 wt. % to 25 wt. % of the alloy. In another embodiment, the zinc ions and the nickel ions are present at concentrations sufficient to deposit a zinc-nickel ternary or higher alloy comprising a nickel content from 8 wt. % to 22 wt. % of the alloy.

In addition to zinc and nickel, the electroplating bath in accordance with the present invention can further include one or more of Te-, Bi- and Sb-ions, and in some embodiments may also include one or more additional ionic species selected from ions of Ag, Cd, Co, Cr, Cu, Fe, In, Mn, Mo, P, Sn and W. Generally, the alloying elements are contained in the deposited zinc nickel alloy in smaller weight percentages only and are preferably provided in the plating bath via its salts. Generally, all water soluble salts are suitable.

In one embodiment, the electroplating baths of the invention contain an acidic component in sufficient quantity to provide the bath with an acidic pH. In one embodiment, the acidic electroplating bath has a pH in the range from 0 to 6.5. In another embodiment, the acidic electroplating bath has a pH in the range from 0.5 to 6, and in another from 1 to 5, and in yet another, from 1 to 3. In one embodiment, the pH of the acidic bath is in the range from 3.5 to 6. In another embodiment, the acidic pH includes any pH up to, but less than 7. A pH range of 0.5 to 6 is preferred.

The acidic electroplating bath may include any appropriate acid, organic or inorganic or appropriate salt thereof. In one embodiment, the acidic electroplating bath comprises one or more of hydrochloric acid, sulfuric acid, sulfurous acid, nitric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, an aromatic sulfonic acid such as substituted or unsubstituted benzene sulfonic acids, toluene sulfonic acid, and similar and related aromatic sulfonic acids, methane sulfonic acids and similar alkyl sulfonic acids, a poly carboxylic acid such as citric acid, sulfamic acid, fluoroboric acid or any other acid capable of providing a suitable acidic pH. The acid itself or an appropriate salt thereof may be used, as needed, e.g., to obtain the desired pH.

The use of acidic zinc and zinc alloy plating bath compositions for a method and in an apparatus according to the present invention is preferred over alkaline plating bath compositions.

The use of acidic zinc and zinc alloy plating bath compositions in combination with an acidic anolyte compartment according to the invention has several technical advantages.

The first advantage is the safety of an apparatus, wherein both compartments are acidic in pH. In case of leakages no strongly exothermic reaction occurs when the solutions of the plating bath as well as the anolyte are within the same pH range, which would happen if the plating bath would be alkaline and the anolyte would be acidic in pH (neutralisation reaction).

Furthermore, when using an acidic anolyte, H₃O⁺ ions are the species migrating from the anolyte compartment through the cationic ion exchange membrane into the plating bath during the metal deposition process. This migration also helps to keep the pH value of the plating bath constant. Usually, a cathodic side reaction during metal plating, particularly when working at a higher current densities, is the evolution of hydrogen gas from acidic solutions. During this reaction H₃O⁺ ions are reduced to hydrogen and the pH of the plating bath would increase. Migrating H₃O⁺ ions from the anolyte into the plating bath, however, help to maintain the pH, which is beneficial.

In contrast, using an alkaline anolyte results in migration of potassium ions (source: KOH) from the anolyte compartment into the plating bath. This results in the undesired accumulation of potassium ions in the plating bath which is as undesired as the accumulation of zinc ions. The same accumulation would happen in case other counter ions for hydroxide are used.

In one embodiment, the electroplating baths of the invention contain an inorganic alkaline component in sufficient quantity to provide the bath with an alkaline pH.

In one embodiment, the amount of the alkaline component contained in the electroplating bath is an amount sufficient to provide a pH of at least 10, and in one embodiment, an amount sufficient to provide a pH of at least 11 or, in one embodiment, a pH of 14. In one embodiment, the alkaline pH is in the range from a pH of 7.5 to a pH of 14.

The alkaline electroplating bath may contain any appropriate base. In one embodiment, the alkaline component is an alkali metal derivative such as sodium or potassium hydroxide, sodium or potassium carbonate, and sodium or potassium bicarbonate, etc., and mixtures thereof.

In one embodiment, the electroplating bath of the invention further comprises one or more complexing agents. In one embodiment the electroplating bath has an alkaline pH. In this case it is required to include a complexing agent to help dissolve and maintain the nickel ions in solution and prevent their precipitation. In an acidic electroplating bath, nickel does not need a complexing agent to remain in solution. It is noted that some of the complexing agents are also listed above as acids useable in the acidic baths.

The use of complexing agents and other organic additives is well known in the art and for example described in document US 2005/0189231 A1.

In the process according to the invention, in one embodiment, the deposition of the coatings is carried out at a current density in the range from about 0.01 to about 150 A/dm², in one embodiment, from about 0.5 to about 25 A/dm² and in one embodiment, from about 1 to about 10 A/dm². The process conveniently may be carried out at room temperature. In one embodiment, the process may be carried out at a temperature in the range from 10° C. to 90° C., and in one embodiment, from 15° C. to 45° C., and in one embodiment, 25° C. to 40° C. The disclosed higher temperatures may be useful, e.g. for inducing evaporation of water from the electrolyte or to provide beneficial deposition conditions.

The apparatus according to the present invention enables the operator to change the dissolution rate of the metal from the at least one soluble anode material to adjust it according to the deposition rate at the cathode and thereby maintain a constant metal ion concentration in the plating bath.

The electrode assembly comprises the at least one soluble zinc anode (2), the cathode (workpiece) (3) and the at least one insoluble anode (5).

In a first embodiment shown in FIG. 1, the soluble anode (2) and the insoluble anode (5) are both connected to the cathode (3) by an adjustable power supply (6). The current at the soluble anode (2) can be adjusted by an adjustable resistor element 7 a and/or the current at the insoluble anode (5) can be adjusted by an adjustable resistor element 7 b.

Alternatively, the current distribution between soluble anode (2) and insoluble anode (5) can be adjusted by varying the distance between soluble anode (2) and the cathode (3) and/or the distance between insoluble anode (5) and the cathode (3). In such a case no resistor element is required.

Additionally, the current distribution between soluble anode (2) and insoluble anode (5) can be adjusted by varying the active surface area of the soluble anode (2) and/or the active surface area of the insoluble anode (5). In such a case no resistor element is required.

In a second embodiment shown in FIG. 2, the soluble anode (2) and the cathode (3) are connected by a first adjustable power supply (6 a) and the insoluble anode (5) and the cathode (3) are connected by a second adjustable power supply (6 b). In this case the current between soluble anode (2) and cathode (3) and between insoluble anode (5) and cathode (3) can be adjusted independently.

In a third embodiment shown in FIG. 3, a second soluble anode (2′) is provided which is the metal ion source of the second metal to be plated. This setup is particularly useful for alloy deposition on the cathode (3). The first soluble anode (2) and the second soluble anode (2′) are both connected to the cathode (3) by a first (6 a) and a second (6 c) adjustable power supply. In this case the current between the soluble anodes (2) and (2′), respectively and cathode (3) and between insoluble anode (5) and cathode (3) can be adjusted independently.

Alternatively, instead of changing the current applied to one of the three anodes mentioned above during a specified time by varying the current, a constant current from one rectifier could be applied to all three anodes via a switching apparatus distributing the current to the different anodes alternatingly.

Preferably, the current distribution between the at least one soluble anode (2) and the insoluble anode (5) is adjusted in a way that the resulting current efficiency at the at least one soluble anode (2) corresponds to the cathode efficiency of the cathode (3). The remainder of the anodic current is directed to the insoluble anode (5) and consumed in a secondary anode reaction. When adjusting the anode assembly in a way that the metal ion concentration in the plating bath remains constant because all of metal ions dissolved at the at least one soluble anode (2) are deposited by reduction on the cathode (3).

In other words, the current of the soluble anode (2) is adjusted by the first adjustable power supply (6 a) to dissolve the amount of ions necessary for the plating of the cathode (3). The additional current of the insoluble anode (5) is adjusted by the second adjustable power supply (6 b) to compensate the lower efficiency, in comparison to the efficiency of the soluble anode, of the cathode (3) to achieve the desired plating quantity at the cathode. When adjusting the anode assembly in such a way that as many ions will be dissolved from the soluble anode (2) as will be plated on the cathode (3), the metal ion concentration in the plating bath remains constant.

Various ion exchange membrane materials are suitable for the necessary separation of the plating bath and the anolyte. Such membrane materials are commercially available and are selected by the expert skilled in the art depending on the plating bath composition, e.g. its pH, the metals ions to be deposited, the temperature etc.

Particularly advantageous are cationic ion exchange membranes if the plating bath pH is acidic, e.g. made from fluorinated polymers like Nafion.

The ion-selective membrane may not only be anionic or cationic, but can also be of bipolar or charge-mosaic type. The anionic membrane may also be referred to as an anion-exchange membrane, and the cationic membrane may also be referred to as a cationic-exchange membrane. A bipolar membrane is an ion-exchange membrane having a structure in which a cationic membrane and an anionic membrane are attached together. A charge-mosaic membrane is composed of a two-dimensional or three-dimensional alternating cation- and anion-exchange channels throughout the membrane. In one embodiment, a combination of an anionic and a cationic membrane is used, with the anionic-selective membrane on the anode side and the cationic-selective membrane on the cathode side. In another embodiment, a combination of an anionic and a cationic membrane is used, with the cationic-selective membrane on the anode side and the anionic-selective membrane on the cathode side. In such combinations of anionic and cationic, the membranes are separated at least slightly during use, in distinction to a bipolar membrane, in which the two membranes are attached together. In one embodiment, the bipolar ion-selective membrane is disposed with its cationic side towards the cathode and its anionic side towards the anode, and in another embodiment, in the opposite configuration. Any known anionic, cationic, bipolar or charge-mosaic membrane may be used, and appropriate membranes may be selected from those known in the art.

Cationic ion selective membranes are particularly preferred.

Exemplary cationic ion selective membranes can be made from materials such as NAFION, perfluorosulfonate ionomers and polyperfluorosulfonic acid; ethylene-styrene interpolymer (ESI) available from Dow Chemical; sulfonate dpolyarylether ketones, such as VICTREX, PEEK, polybenzimidazole, available as PBI from Celanese GmbH.

The solution in the second compartment can correspond to the plating bath of the first compartment. Alternatively and preferably, it is for example an acid like diluted (e.g. 5 wt. %) sulfuric acid or hydrochloric acid.

The insoluble (also denoted inert) anode material inside the second compartment is for example a titanium anode, a platinised titanium anode or a ceramic coated anode or a graphite or any other carbon electrode. Particularly preferred is a platinised titanium mesh metal anode.

The invention is further illustrated by the following non-limiting examples.

EXAMPLES Example 1 (Comparative)

A zinc nickel plating bath is provided containing 20 g/l of zinc ions, 30 g/l of nickel ions (both added as their chloride salts) and potassium chloride to establish a total chloride ion concentration of 160 g/l chloride as well as 20 g/l boric acid, 2 g/l polyalkyleneimine, 2 g/l of an aromatic carboxylic acid, 0.4 mol/l acetate, 20 mg/l benzylidene acetone, 8 g/l of an anionic polyoxyalkylated surfactant and 1 g/l of a polyoxyalkylated nonionic surfactant. The process is run using a soluble zinc anode and a soluble nickel anode, but no ion exchange membrane. The anodic current is being distributed between both anodes in an 5:1 (Zn:Ni) ratio at an anodic current density of 2 A/dm² and a cathodic current density of 2 A/dm² with mild steel workpieces on a cathode rack. The process is run at 35° C. and at pH of 5.3 with mechanical cathode agitation for 30 min.

The metal at the cathode is deposited showing a smooth, glossy coating of an average thickness of 13.8 μm having 14 wt. % of nickel homogenously distributed within the deposit. The plating thickness corresponds to a cathodic current efficiency of 90%. The thickness of the deposit on a rectangular shaped workpiece taken from the center of the rack is 20 μm on the corners, 7 μm in the center and 10 μm in between center and corner. The results correspond to a plating bath without ageing.

Example 2 (Comparative)

The same plating procedure as described in Example 1 using the same bath is conducted several times in the same bath with a drag-out of 150 ml/m² bath and a drag-in of 150 ml/m² of water achieving an overall throughput of 2.500 kAh. Replenishment is performed to compensate for drag-out losses.

The zinc and nickel concentrations are monitored and an increase of both metals can be detected, reaching 28 g/l Zn-ions and 35 g/l Ni-ions after 2500 kAh. The pH is adjusted by addition of hydrochloric acid or potassium hydroxide in order to keep the nickel concentration in the alloy constant. By these means, a smooth, glossy deposit with a homogenous nickel incorporation of 14 wt. % of nickel having an average deposit thickness of 13.8 μm is achieved. After 2500 kAh, the thickness of the deposited zinc-nickel layer on a rectangular shaped workpiece taken from the center of the rack is 25 μm on the corners, 5 μm in the center and 7 μm in between corner and center.

Thus, a plating bath without ion exchange membrane system does not provide a homogeneous thickness distribution after prolonged plating (ageing).

Example 3 (According to the Invention)

The same plating procedure as described in Example 2 using the same bath make-up and replenishment is conducted up to 2500 kAh. The plating bath contains an cationic exchange membrane with a Nafion membrane to separate the anolyte from the plating bath (catholyte). The corresponding setup is shown in FIG. 3. A carbon anode is used, the anolyte is a 5 wt. % aqueous sulfuric acid solution. A separate rectifier is used for the anolyte. The rectifiers used on the zinc and nickel anode remain in place with their cathodes connected together with the cathode from the now added third rectifier to the workpieces. The anode of the third rectifier is connected to the carbon anode in the membrane compartment. The current is adjusted so that 5% of the current is run through the new carbon anode in the membrane anode compartment, 79% on the zinc anode and the remaining 16% on the nickel anode. After 2500 kAh the process still yields a smooth and glossy deposit of 13.8 μm average thickness and 14 wt. % Ni content in the deposit. The zinc and nickel ion concentrations in the bath remained constant at 20 g/l zinc ions and 30 g/l of nickel ions. After 2500 kAh, the thickness on a rectangular shaped workpiece taken from the center of the rack is 20 μm on the corners, 7 μm in the center and 10 μm in between corner and centers, thus indicating maintained thickness distribution and deposit quality even after prolonged plating. 

1. Apparatus for the electrolytic deposition of a zinc or zinc alloy layer on a workpiece, the apparatus comprising: a) a container to hold the zinc or zinc alloy plating bath which comprises zinc ions; b) an electrode assembly in the zinc or zinc alloy plating bath, the assembly comprises at least one soluble zinc anode to provide zinc ions to be deposited and a cathode corresponding to the workpiece to be zinc or zinc alloy plated and an enclosure defining an anolyte compartment and bearing on at least a portion thereof an ion exchange membrane in contact with the plating bath which comprises an acid and the electrode assembly further comprises an insoluble anode, and c) at least one power supply connecting the electrode assembly and to provide an electric current for zinc or zinc alloy deposition, wherein the zinc or zinc alloy plating bath and the anolyte compartment have about the same pH value of between 0.5-6.
 2. Apparatus according to claim 1, comprising at least one means for adjusting the power supply that allows adjustment of the current distribution between the soluble zinc anode and the insoluble anode in order to determine the zinc ion dissolution rate of the soluble zinc anode.
 3. Apparatus according to claim 1, wherein the electrode assembly comprises a second soluble anode selected from the group consisting of nickel, cobalt, iron, chromium, cadmium, tin, copper, silver, gold, platinum, palladium, manganese, ruthenium, rhodium, iridium, osmium, rhenium, tungsten, molybdenum, vanadium, indium, bismuth, antimony, selenium, germanium, gallium, tantalum and niobium.
 4. Apparatus according to any of the foregoing claims claim 1, wherein the first soluble zinc anode, the second soluble anode and the insoluble anode are equipped individually with separate power supplies.
 5. Apparatus according to claim 1, wherein the ion exchange membrane is a cationic ion exchange membrane.
 6. (canceled)
 7. Apparatus according to claim 1, wherein the acid in the anolyte compartment is selected from hydrochloric acid and sulfuric acid.
 8. (canceled)
 9. (canceled)
 10. Apparatus according to claim 1, wherein the workpiece is a steel based material.
 11. Apparatus according to claim 1, wherein the insoluble anode is selected from the group consisting of titanium anodes, platinized titanium anodes, ceramic coated anodes, graphite anodes and carbon based anodes.
 12. (canceled)
 13. Method for electrolytic deposition of zinc or zinc alloys in an apparatus according to claim 1 comprising the following steps: a) providing in the container a zinc or zinc alloy plating bath containing additives and an acid b) depositing a zinc or zinc alloy layer on the cathode corresponding to the workpiece to be metal plated by providing an electrical current through the adjustable power supply.
 14. Method according to claim 13, wherein the anode efficiency of the soluble zinc anode is adjusted to correspond to the cathode efficiency of the cathode and the remainder of the anodic current is directed to the insoluble anode and consumed in a secondary anode reaction.
 15. Method according to claim 13, wherein the zinc alloy plating bath is a zinc nickel alloy plating bath and comprises organic additives selected from surfactants and complexing agents, a soluble nickel salt and a soluble zinc anode.
 16. Method according to claim 14, wherein the zinc alloy plating bath is a zinc nickel alloy plating bath and comprises organic additives selected from surfactants and complexing agents, a soluble nickel salt and a soluble zinc anode.
 17. Apparatus according to claim 2, wherein the electrode assembly comprises a second soluble anode selected from the group consisting of nickel, cobalt, iron, chromium, cadmium, tin, copper, silver, gold, platinum, palladium, manganese, ruthenium, rhodium, iridium, osmium, rhenium, tungsten, molybdenum, vanadium, indium, bismuth, antimony, selenium, germanium, gallium, tantalum and niobium.
 18. Apparatus according to claim 2, wherein the first soluble zinc anode, the second soluble anode and the insoluble anode are equipped individually with separate power supplies.
 19. Apparatus according to claim 2, wherein the ion exchange membrane is a cationic ion exchange membrane.
 20. Apparatus according to claim 2, wherein the insoluble anode is selected from the group consisting of titanium anodes, platinized titanium anodes, ceramic coated anodes, graphite anodes and carbon based anodes.
 21. Apparatus according to claim 3, wherein the first soluble zinc anode, the second soluble anode and the insoluble anode are equipped individually with separate power supplies.
 22. Apparatus according to claim 3, wherein the insoluble anode is selected from the group consisting of titanium anodes, platinized titanium anodes, ceramic coated anodes, graphite anodes and carbon based anodes. 